1. Field of the Invention
The present invention generally relates to a ballast, and more particularly relates to a ballast for a cold cathode fluorescent lamp (CCFL).
2. Description of Related Art
Liquid crystal displays (LCDs) are used in a variety of environments ranging from televisions to computers. Cold cathode fluorescent lamps (CCFL) are common light sources used in the LCDs, because of their high brightness, low power consumption and low heat-generation.
Ballasts are used for controlling the CCFL during startup and operation. Typically, a ballast includes an oscillator, a drive circuit, a half-bridge inverter, and a resonant LC circuit. The oscillator is used for generating a series of pulses, and applying the pulses to the drive circuit. The drive circuit is configured for outputting two drive signals to the half-bridge inverter on receiving the pulses. The two drive signals are applied to two field effect transistors (FET) in the half-bridge inverter, for driving the two FETs, to be turned on alternatively. The half-bridge inverter outputs a square wave accordingly. The square wave is applied to the resonant LC circuit, thus the resonant LC circuit sends a high-level signal, for driving the CCFL to start to work.
The output of the half-bridge inverter directly depends on a non-overlapping time of the two drive signals, further affecting the startup of the CCFL. However, as the drive circuit outputs the drive signals without any feedback, it is difficult to adjust the non-overlapping time of the two drive signals, thus the non-overlapping time of the two drive signals may not be consistent with each other. Furthermore, as there are a lot of external and internal interferences and noise, the non-overlapping time becomes unstable, which causes difficulty in the starting of the CCFL.
Therefore, it is an object of the present invention to provide a kind of ballast which is able to stably drive the CCFL.